Over the past few years medical treatments have advanced dramatically and it is now possible to treat, on a continuous or at least semi continuous out-patient basis, various complaints and disorders which were previously not treatable at all or which required lengthy stays in hospital so that skilled personnel were available to administer relatively potent and possibly toxic drugs at prescribed time intervals. Various ambulatory medication infusion pumps have been developed over the years but most have been relatively bulky so that they cannot be worn discreetly beneath the clothes or have been relatively expensive. Efforts have, therefore, centred upon miniaturization and cost reduction. The problems of miniaturization have been largely solved by extensive use of solid state electronic devices to control and time the administration of dose amounts of the medication, by advances in battery technology which has reduced the size of the power source required and by the use of simple spring devices to provide a pressurized medication reservoir. Attention is directed to our earlier patent, U.S. Pat. No. 4,596,558 issued June 24, 1986 which describes such a miniature ambulatory medication infusion device. The cost of this device has been reduced relative to devices presently on the market by the simplification of parts and extensive use of polymeric materials for manufacture. Problems do, however, still remain with respect to the cost of disposable elements, reliability. simplicity of use and most importantly the provision of a multi mode fail-safe performance. In our prior device a three way valve is provided between a relatively large medication reservoir pressurized by means of a pair of external leaf springs and a smaller medication dispenser also pressurized by means of external leaf springs. The pressure in the medication reservoir is greater than the pressure in the medication dispenser, which in turn is greater than the venous pressure. In operation, the three way valve, which in the normal de-energized state blocks flow from the medication dispenser to the patient, is energized to open a passage from the medication dispenser to the patient while blocking the passage from the medication reservoir. As the pressure of the medication dispenser is greater than the venous pressure the dispenser discharges a metered dose intravenously or subcutaneously to the patient. Upon deactivation, the valve returns to the rest position with the patient passage blocked but the passage from the medication reservoir to the medication dispenser open. As the pressure in the reservoir exceeds that of the dispenser, the dispenser refills to capacity. The problem is that through spring or solenoid failure or the presence of foreign bodies in the system, it is conceivable that the three way valve could come to rest in an intermediate position in which none of the ports are properly sealed. This would lead to a condition known as streaming, in which medication would flow continuously and directly from the relatively large reservoir to the patient, with the obvious risk of a dangerous overdose, until the reservoir is completely voided or until some corrective action is taken.